It is well known that internal combustion engines can produce undesirable chemical species in their exhaust streams. It is therefore desirable to eliminate or at least reduce such pollutants to levels low enough that human health is not adversely affected. As a result of the high temperatures that are reached during a combustion event, many chemical species are produced from the combustion of hydrocarbon fuels in air, including the oxides of nitrogen (NO and NO2, collectively referred to as NOx). Due to their impact on human health, many countries have enacted legislation that seeks to limit the emission of NOx from both mobile and stationary sources, and many techniques have been developed to achieve this objective. Among these, the use of catalyst technology has been found to be particularly effective and economically viable.
The remediation of NOx for lean-burn engines requires the addition of a reductant in conjunction with a suitable catalyst. For example, the reduction of NOx requires near real-time reductant dosing control since NOx production closely follows engine load but is moderated by the amount of ammonia already stored on the catalyst. Accordingly, the reductant dosing schedule is a highly dynamic activity. One effective technology for the remediation of NOx in an oxygen-rich exhaust stream is the technique widely known as Selective Catalytic Reduction (hereafter referred to as SCR). In this approach, an ammonia-containing reagent (or reductant) is injected into an exhaust stream at a rate closely related to the instantaneous NOx content of that stream wherein the ammonia (NH3) reacts with the NOx in conjunction with a zeolite-based or similar catalyst such that the pollutant is converted to harmless nitrogen (N2) and water.
A known exhaust system for an internal combustion engine 8 is shown in FIG. 1. The exhaust system includes an exhaust pipe 12 into which exhaust from the engine 8 is received before transfer to a downstream exhaust pipe outlet 14. The exhaust pipe is provided with a diesel oxidation catalyst (DOC) 10 and a diesel particulate filter (DPF) 16 arranged upstream of an SCR catalyst 18. The DOC 10 removes carbon monoxide and hydrocarbons from the exhaust stream 12. The DPF 16 removes diesel particulate matter and soot from the exhaust stream 12. An injector 20 for the reductant is located upstream of the SCR catalyst 18, between the DPF 16 and the SCR catalyst 18. The injector 20 delivers a controlled dose of reductant, such as urea, into the exhaust stream just upstream of a mixer 22 and the SCR catalyst 18. The ammonia in the exhaust flow reacts with the NOx in conjunction with the zeolite based or similar catalyst so that harmless nitrogen (N2) and water is emitted at 14.
One drawback of such a system is that the temperature of the SCR catalyst 18 must be hot enough for it to become effective. On engine start-up, before the SCR catalyst 18 has had chance to heat up, this can lead to problems where insufficient NOx is converted, so that emissions levels are higher than is desirable and to the extent that emissions legislation in some territories may not be satisfied for a period of engine operation. In some engines it is possible to locate the SCR catalyst 18 closer to the engine 8 so as to improve thermal heating of the catalyst, but this solution is not always effective and, in any case, certain vehicle architectures do not suit the reconfiguring of hardware in this way.
It is one object of the present invention to provide an exhaust system, an exhaust control system and a method of controlling an exhaust system which substantially overcomes or mitigates the aforementioned problems.